operating problems in anaerobic digestion plants … · operating problems in anaerobic digestion...

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Waste Management 27 (2007) 30–43

Operating problems in anaerobic digestion plants resultingfrom nitrogen in MSW

Klaus Fricke a, Heike Santen a,*, Rainer Wallmann b, Axel Huttner b, Norbert Dichtl c

a Technical University of Braunschweig, Leichtweiß-Institute, Department of Waste Management, Beethovenstr. 51a, 38106 Braunschweig, Germanyb IGW – Ingenieurgemeinschaft Witzenhausen Fricke and Turk GmbH, Bischhauser Aue 12, 37213 Witzenhausen, Germany

c Technical University of Braunschweig, Institute for Sanitary Engineering, Pockelsstr. 2a, 38106 Braunschweig, Germany

Accepted 7 March 2006Available online 24 July 2006

Abstract

Organic waste and municipal solid waste usually contain considerable amounts of different nitrogen compounds, which may inhibitanaerobic degradation processes and cause problems in the downstream and peripheral devices. This refers particularly to the differentprocess stages of anaerobic digestion, to wastewater treatment, and to exhaust air treatment.

Neither the knowledge about nitrogen problems nor the technologies for elimination of nitrogen compounds from the wastewater orthe exhaust air of anaerobic digestion can be regarded as state-of-the-art. Most of the technologies in question have already been appliedin other areas, but are barely tested for application in anaerobic digestion plants. The few performance data and experiences at hand weremainly derived from pilot and demonstration facilities.

In this paper, the problem of nitrogen will be discussed in detail according to the separate problem fields based on the authors’ expe-rience, as well as on the basis of a review of the relevant literature. Furthermore, possible solutions will be proposed and the need forfurther research and development will be formulated.� 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Due to the relatively low costs, the high flexibility of theprocess and the possibility of centralized and decentralizedapplication, mechanical-biological waste treatment (MBT)processes are gaining importance, not only in Germany. Inthis context, anaerobic digestion for municipal solid wastetreatment is becoming increasingly interesting due to itsadvantages in terms of energy production and exhaustemissions compared to aerobic procedures. Nevertheless,anaerobic digestion has not yet been able to establish itselfon the market to the same extent as aerobic technologies.Apart from the higher investment costs for anaerobicdigestion plants in comparison to aerobic treatment plants,this is also due to the fact that anaerobic digestion is stillconsidered to be less stable in operation. Moreover, opera-

0956-053X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2006.03.003

* Corresponding author. Tel.: +49 531 391 3958; fax: +49 531 391 4584.E-mail address: [email protected] (H. Santen).

tional problems are more difficult to remedy, once theyhave occurred.

One important source of operational problems is thenitrogen compounds, which enter the process with the feedmaterial. Table 1 shows the nitrogen content of differentorganic waste, sewage sludge and municipal solid waste.In the untreated raw waste, the nitrogen is predominantlyorganically bound.

Nitrogen may cause problems in anaerobic digestionbecause of its metabolic products:

� Ammonia (NH3),� Ammonium ðNHþ4 Þ,� Dinitrous oxide (N2O),� Nitrite ðNO�2 Þ,� Nitrate ðNO�3 Þ.

Fig. 1 shows the process areas and material stages inwhich nitrogen compounds may cause problems.

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Table 1Physical–chemical parameters and nutrient contents of selected organic waste materials

H2O (% FM)b Organic drymatter (% DM)c

Ntotal (% DM)c P2O5 (% DM)c K2O (% DM)c CaO (% DM)c MgO (% DM)c

Organic waste 52–80 34–81 0.6–2.1 0.3–1.5 0.6–2.1 2.2–6.8 0.2–1.7Green waste (‘‘soft’’ organic) 48–80 32–70 0.3–1.9 0.4–1.4 0.4–1.6 0.7–7.4 0.3–1.2Green waste (tree cuttings) 25–52 65–85 0.1–0.4 0.1 0.3–0.5 0.5–1 0.1–0.15Sewage sludge (digested) 65–85 15–40 4.0–5.3 4.7–5.2 0.3–0.5 5.7–8.2 0.8–1.2Bark 45–75 60–85 0.2–0.6 0.1–0.2 0.3–1.5 0.4–1.3 0.1–0.2Kitchen waste 75–95 1.0–1.5Grape pomace 75–75 1.5–2.5 0.8–1.2 3.4–5.3 1.4–2.4 0.21Fruit pomace 70–80 90–95 1.1 0.2– 0.6 1.57 1.1 0.2Rumen contentsa 10–20 80–90 1.3–1.2 1.1–1.6 0.5–0.6 2.0 0.6Paper 25–30 62–79 0.2–0.8 0.15–0.6 0.02–0.1 0.5–1.5 0.1–0.4Draff/masha 90–95 90–95 0.8–1.8Yeast residues 40–60 90–95 1.4–2.0Residual household waste

(after separate collection)35–45 50–70 0.7–1.3 0.8–1.4

a Kuhn (1995) and Weiland (1999).b FM, fresh matter.c DM, dry matter.

Anaerobic Digestion

DewateringAerobic

post-treatment

Pre- Treatment Conditioning Conditioning Landfilling

Biogas

Incineration plant, RDF

Wastewater

Ammonianitrous oxide,

Ammonium/Ammonia

Ammonium/ Ammonia

Ammonium

Ammonium

Exhaust air

Fig. 1. Process areas and material stages of an anaerobic digestion plant possibly affected by nitrogen problems.

K. Fricke et al. / Waste Management 27 (2007) 30–43 31

2. Biological process

The anaerobic digestion of organic matter is a complexprocess, which falls into four degradation steps. The spe-cific microorganisms that take part in the process have dif-ferent requirements on environmental conditions andmoreover coexist in synergetic interactions. Nitrogen playsan important role in anaerobic digestion: Nitrogen is neces-sary for the formation of new biomass. Furthermore, in theform of ammonium, nitrogen contributes to the stabilisa-tion of the pH value in the reactor. However, ammoniumin high concentrations may lead to the inhibition of thebiological process.

Microorganisms need nitrogen for the production ofnew cell mass, the absorption of nitrogen taking place inthe form of ammonium. The nutrient requirement is low,which is due to the low biomass formation. A nutrient ratioof the elements C:N:P:S at 600:15:5:3 is sufficient for meth-anisation. As the reduced nitrogen compounds are not

eliminated in the process, the C/N in the feed materialplays a crucial role. The C/N should range from 20 to 30in order to ensure sufficient nitrogen supply for cell produc-tion and the degradation of the carbon present in theprocess, and in order to avoid at the same time excess nitro-gen, which could lead to toxic ammonium concentrations(Weiland, 2001).

Ammonium is an important parameter for the buffercapacity in an anaerobic reactor. With concentrations ofup to 1000 mg/l, ammonium stabilises the pH value(ATV, 2002). Ammonium is released during the anaerobichydrolysis of organic nitrogen compounds, causing anincrease of the pH value. The ammonification thus coun-teracts the reduction of the pH value resulting from theacidification step of anaerobic digestion (ATV, 1993).

At a sufficiently high concentration, almost allsubstances inhibit anaerobic digestion (ATV, 1990). Itshould be noted that only the undissociated form of theintermediate catabolic product has an inhibiting effect on

Fig. 2. Dissociation balance between ammonia/ammonium depending on pH and on temperature (calculated according to Kollbach et al., 1996).

32 K. Fricke et al. / Waste Management 27 (2007) 30–43

microorganisms. As the dissociation equilibrium dependson the pH and on temperatures in the reactor, which bothmay vary, it is difficult to provide detailed data on toxic orinhibiting threshold concentrations. The dissociation bal-ance of ammonia and ammonium, for instance, changesto ammonia with an increasing pH value and temperatureas shown in Fig. 2. From this follows that even smallchanges in the pH value are sufficient to cause an inhibi-tion. Furthermore, the bacteria may adapt themselves tohigh concentrations of certain substances, as long as theconcentration of the respective substance increases slowly.Because of this situation, it is difficult to determine an exactthreshold concentration that inhibits the process; ratherbroad ranges of possibly inhibiting concentrations can begiven.

The ammonia-induced inhibition occurs primarily dur-ing the anaerobic digestion of organic waste materials,which are rich in proteins, as ammonia nitrogen is releasedthrough the mineralisation of organic nitrogen compounds.The range of inhibiting concentrations of ammonia isbetween 30 and 100 mg/l (at pH value 6 7 and tempera-ture 6 30 �C), whereas the respective concentrations ofammonium are between 4000 and 6000 mg/l (ATV, 1990).

The inhibition effects by different intermediate catabolicproducts can counteract each other. With an increasing pHvalue, for instance, the inhibition by hydrosulphide and byvolatile fatty acids declines, whereas the inhibition byammonium nitrogen increases. With the presence of certainsubstances, the inhibition impact may even be reversed. Inthe presence of hydrosulphide and carbon dioxide, forinstance, the dissociation balance of ammonia/ammoniumis displaced in the direction of ammonium and the inhibi-tion by ammonia is reversed (Knoche et al., 1996).

As mentioned above, microorganisms have the ability toadapt themselves to varying environmental conditions dur-ing a slow increase of, say, the ammonium or ammoniaconcentration in the reactor. Nevertheless, a suddenincrease in the ammonia concentration leads to an inhibi-

tion of the biological process. There are different emer-gency measures for the rapid recovery of the process, asfor example stopping the substrate supply, the additionof substrate with low nitrogen content, refeeding ofdigested material or lowering the pH value by addition ofacids. All of these measures can only eliminate low degreesof inhibition. In the case of a high degree of inhibition, theonly option is to empty the reactor and re-initiate the pro-cess. Therefore, close monitoring of the process is indis-pensable for early identification of inhibition effects.

In anaerobic digestion processes with intensive processwater recirculation, ammonium may accumulate in theprocess water and thus in the substrate for anaerobic diges-tion with the effects described above. In that case, furthermeasures for ammonium elimination may be necessary(see Section 4).

3. Exhaust air

The exhaust air emissions from waste treatment plantsplay a key role with regard to the acceptance by the popu-lation and the ecologic evaluation of the process. Nitrogencompounds that are relevant to the quality of the exhaustair are primarily ammonia and dinitrous oxide. In somecountries, exhaust air emission quality and particularlyemissions of odour and dinitrous oxide may be subject topermits.

In Germany, there are different legal licensing guidelinesfor the recovery of organic waste (‘‘biowaste’’), on the onehand, and for the treatment of municipal solid waste on theother:

� For the construction and operation of plants for organic

waste treatment there are only essential requirements onexhaust air emission control, e.g., a minimum 300-m dis-tance of these plants from populated areas and defini-tion of maximum odour emissions of 500 odour units/m3 (German Technical Instruction on Air Quality


Control -TA-Luft; Anonymous, 2002). These require-ments can easily be fulfilled by structural measuresand the treatment of the exhaust air from aerated win-drows in a biofilter.� In comparison to that, legal requirements on exhaust air

emission control from municipal solid waste treatmentare more detailed and stricter. The German 30th FederalEmissions Control Act (Anonymous, 2001) requires thecomplete encapsulation of the mechanical biologicalwaste treatment plants, including exhaust air collectionand treatment; it also defines limit values for some airpollutants, such as TOC, N2O and others (Table 2). Inorder to meet all of these requirements, a simple exhaustair purification in biofilters is not sufficient. As shown byresearch results and up-to-date operating experiences, itis necessary to treat the exhaust air in a thermal regen-erative oxidation plant (TRO) in combination with anacid scrubber (Wallman et al., 2001).

The relevant exhaust air emissions of nitrogen com-pounds are ammonia and dinitrous oxide emissions. Whileammonia contributes to the odour emissions and causesadverse effects on humans, the dinitrous oxide contributesto the anthropogenic greenhouse effect. Dinitrous oxideemissions can occur both during the intensive thermophilicphase of aerobic treatment/composting, and also duringthe aerobic post-treatment of solid digestion residues; thisis equally relevant for the treatment of organic and residualwaste.

Due to the mineralisation of organic nitrogen com-pounds under anaerobic conditions, the total nitrogen inthe solid digestion residue is mainly present as ammoniumand ammonia. In the first phase of aerobic post-treatment,most of it is stripped out in the form of highly volatileammonia. The reduction of the ammonia concentrations

Table 2Exhaust air emissions in comparison with the German limit values according2001)

German limit value(according to 30. BImSchV

Exhaust air (m3/ton waste input)(to treatment plant)

TOC (mg/m3)a 20/40b

TOC (g/Mg) 55Dinitrous oxide (g/ton waste input) 100Odour (odour units/m3)a 500

Polychlorinated dibenzo-p-dioxins(PCDD) and polychlorinateddibenzofurans (PCDF) (ng ToxicityEquivalents TE/m3)a

0.1

Dust (mg/m3) 30/10a

Ammonia (mg/m3)a

a Referring to standard cubic meters (0 �C, 1013 bar).b Daily/half-daily mean.

in the waste by stripping with exhaust air leads to anincreased release of ammonia from the ammonium frac-tion, as the proportion of ammonia and ammonium is inbalance (see Fig. 2). In this way, up to 25% of the totalnitrogen in the waste may be stripped out into the exhaustair.

The main ammonia emissions take place during the firstweek of post-treatment and can amount to up to 1000 mg/scm (standard cubic meters), as shown by the investigationsof the aerobic post-treatment of digestion residues from aValorga plant (IGW, 2001). In that investigation, theammonia emissions declined in the course of further treat-ment and were about 100 mg/scm by the end of the thirdweek. Similar evolutions of the ammonia concentrationin the exhaust air were detected in other comparable inves-tigations (Fricke et al., 2001).

Very high peak emissions of ammonia may occur if thetemperatures in the windrows are high or if increasedammonium loads enter the aerobic post-treatment as aconsequence of high ammonium contents in the solid diges-tion residue itself or in the process water used for the irri-gation of the windrows. If the aeration is insufficient, highammonia concentrations in the atmosphere of the treat-ment hall may result that exceed the German limit valuefor exposure at working places of 50 ppm. According tocurrent operational experiences, only a strong, optimisedsuction aeration proved to be suitable in order to maintainthe contamination of the atmosphere in the treatment hallbelow critical levels.

The ammonia in the exhaust air may be effectivelyremoved through the use of an acid scrubber; the reductionrate can be close to 100%. Sulphur or nitric acids can beused as scrubbing acids, so that solutions of ammoniumsulphate or ammonium nitrate are formed as products.So far, there is no established market for these products.

to the 30th Federal Emissions Control Act (30. BImSchV – Anonymous,

)Exhaust air beforetreatment(aerobic treatment/MSW composting)

Exhaust air before treatment(aerobic post-treatment ofanaerobic digestion residues)

5000–9000 2000–6000

50–200 (maximumup to 1000)


400–800 200–600<100 <10010,000–30,000 (maximumup to 100,000)

10,000–30,000 (maximumup to 100,000)

�0.1 �0.1

�10 �1030–100 (maximumup to 500)



Nevertheless, the application as fertilizers in agriculture,given a sufficiently high nitrogen concentration, or theapplication for exhaust gas scrubbing during waste inciner-ation are possible recycling options.

Thermal regenerative oxidation (TRO), which is appliedfor the oxidation of the organic carbons in the air, alsoleads to a reduction of ammonia. The reduction yield ofammonia is increased with higher temperatures in the com-bustion chamber of the incineration plant. Yet, as a conse-quence of ammonia incineration, the concentrations ofnitrogen dioxides increase at the same time in the cleangas of the thermal plant (Wallman et al., 2001), leadingto considerable odour emissions in the clean gas. In thatcase, the German limit value for odour emissions of 500odour units/scm is usually exceeded. Acid ammonia scrub-bing before thermal treatment of the exhaust air is thusindispensable in order to limit odour emissions to accept-able values.

As dinitrous oxide contributes to the anthropogenicgreenhouse effect, special attention should be given to dini-trous oxide emissions of waste treatment plants. In Ger-many, the 30th Federal Emissions Control Act defines alimit value for dinitrous oxide emissions (Table 2). Depend-ing on the source, a distinction can be made into the fol-lowing types of dinitrous oxide emissions:

� Primary dinitrous oxide (in the exhaust air before treat-ment, formed during aerobic treatment) and� Secondary dinitrous oxide (newly formed during ther-

mal exhaust air treatment).

Primary dinitrous oxide may be formed in the aerobicpost treatment of the solid digestion residues in the courseof the microbiological degradation of organic matter.

It can thus be produced as an intermediate product ofboth nitrification and denitrification:

� Organic matter is decomposed by heterotrophic micro-organisms, whereas the available concentration ofammonium increases with the degradation of theorganic matter. This mineralisation of the organicallybound nitrogen to ammonium takes place during anaer-obic digestion and can be regarded to be a fairly com-pleted process. The heterotrophic microorganisms inaerobic post-treatment need small amounts of ammo-nium nitrogen for the formation of new biomass, whilethe rest is freely available as ammonium nitrogen. Aconsiderable amount of nitrogen is stripped off in theform of ammonia with the exhaust air. In the courseof aerobic decomposition, the available carbon isreduced, thus the microbial activity of the heterotrophicmicroorganisms decreases as well. Consequently, thetemperature in the windrows drops. The autotrophicnitrification bacteria grow preferably at lower tempera-tures, oxidizing or nitrifying the available ammoniumnitrogen via nitrite and nitrate. However, oxidation ofammonium nitrogen to nitrate can be inhibited by high

ammonia and nitrite concentrations. In particular, inhi-bition is aided by pH values over 7 and high ammoniumconcentrations in the feed material. Due to that, nitritemay accumulate, accompanied by the formation of dini-trous oxide (Wallmann et al., 2003).� Dinitrous oxide can also be formed during denitrifica-

tion, that is, during the reduction of the nitrate to ele-mental nitrogen. The formed dinitrous oxide is notinevitably emitted, because while passing through thewindrow it can be further reduced to elemental nitrogen.However, this requires easily available carbon, which bythe end of the aerobic treatment has almost completelybeen consumed.

In practice, it is very difficult to detect the origin of dini-trous oxide emissions if they occur. Typically, dinitrousoxide emissions are relatively low in the first two weeksof aerobic treatment. This is based on exhaust air measure-ments made by the authors at different aerobic treatmentfacilities of solid digestion residues in Germany (Fig. 3).High ammonia emissions at this stage of aerobic post-treat-ment may, however, lead to so-called secondary formationof dinitrous oxide in the thermal treatment of the exhaustair, as long as there is no acid scrubbing before thermaltreatment. By the end of the second week of aerobicpost-treatment, the concentrations of ammonia and dini-trous oxides in the exhaust gas show a reversed evolution:ammonia emissions decline while dinitrous oxide emissionsincrease.

In order to avoid these increasing dinitrous oxide emis-sions, the followings measures are recommended:

� During the first ‘‘intensive’’ phase of aerobic post-treat-ment, where the temperatures in the windrows increaseto more than 60 �C, the digestion residues should beintensively aerated and turned as frequently as possible(aerobic stabilisation). The ammonium in the waste isthus stripped off as ammonia and can then be eliminatedin an acid scrubber before thermal exhaust air treat-ment. This part of ammonium is no longer availablefor nitrification, so that less dinitrous oxide may be pro-duced. Furthermore, the activity of the methanogenicmicroorganisms, which are present in the waste afteranaerobic digestion, can be additionally reduced byintensive ventilation and regular turning.� The frequency of turning can be reduced when the wind-

row temperature drops below 45 �C. However, whendoing so the gas must be monitored for possible meth-ane formation.

In conclusion it can be said that significant emissions ofdinitrous oxide, which exceed the German limit value of100 g N2O/ton waste input, may occur if process controland monitoring are inadequate. With process control asdescribed above, dinitrous oxide emissions should be keptfar below critical levels as, for instance, defined by the Ger-man limit value.

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7 8 9 10

Duration of aerobic post-treatment

mg/m³

0

20

40mg/l Eluate

NH3 (mg/m³) in exhaust air 1)

N2O (mg/m³) in exhaust air 1)

NH4-N (mg/l) in solid matter

NO3-N (mg/l) in solid matterNO2-N (mg/l) in solid matter

Turning

Fig. 3. Dinitrous oxide concentration in the exhaust gas (before treatment) and nitrogen compounds in the treated waste in the course of the aerobic post-treatment of solid digestion residues (Wallmann et al., 2003). 1 Referring to standard cubic meters (0 �C, 1013 bar).


4. Wastewater

In contrast to simple composting or aerobic treatment,relevant amounts of wastewater are generated duringanaerobic digestion. It should be noted that the quantityof wastewater is basically independent of the type of pro-cess applied, but is determined by the moisture content ofthe waste input and the degree of dewatering after anaero-bic digestion – unless no extra water is added to theprocess.

During anaerobic treatment of organic waste, approxi-mately 200–500 l wastewater/ton waste input are generated(Loll, 1994; Gessler and Keller, 1995; Kubler, 1996).

In comparison to organic waste (‘‘biowaste’’), municipalsolid waste and residual waste (after a separate biowaste col-lection) have as a general rule lower moisture contents,resulting in lower wastewater amounts of approximately100–170 l/ton waste input (Fricke et al., 2004).

Tables 3 and 4 show selected analytical data of wastewa-ter and process water from anaerobic digestion plants formunicipal solid waste, residual waste and organic waste.

In many cases, wastewater generated during anaerobicdigestion is recycled in the process. Nevertheless, the recy-cling of wastewater in the process is limited, on the onehand, by the quality of the recycled water. On the otherhand, there will always be excess water as long as the watercontent of the waste output is higher than the water con-tent of the waste input. Hence, all anaerobic processes haveto integrate some solution for the treatment or disposal ofwastewater.

� There are no specific requirements on the treatment ofwastewater from organic waste treatment neither in theEuropean nor in the German legislation. Up to now,

no special attention has been drawn to the treatmentof this wastewater, as most of it was recycled as fertilizerin agriculture or was sent to municipal wastewater treat-ment plants.� Wastewater from municipal and residual waste treat-

ment will thus usually require further treatment beforedisposal in water courses or in municipal wastewatertreatment plants. Specific requirements might be definedin local legislation. In Germany, the Ordinance on theTreatment of Wastewater from Landfilling of Waste(Rahmen-Abwasser-Verwaltungsvorschrift fur die Reini-

gung von Abwasser aus der oberirdischen Ablagerungvon Abfallen) contains an annex on wastewater fromMBT (Table 4). Most of the limit values defined therecannot be achieved without further treatment.

So far, only little consolidated experience with thedesign and operation of wastewater treatment in anaero-bic digestion plants is at hand. Besides recycling in agri-culture and further treatment in municipal wastewatertreatment plants, different technologies known from sani-tary engineering, such as ultra filtration, reverse osmosis,activated sludge and ammonia stripping, have beenapplied. In the context of a research project on the eco-logically sound disposal of liquid manure funded by theGerman Federal Ministry of Education and Research,the applicability of these technologies for full or partialtreatment of wastewater from organic and agriculturalwaste treatment was systematically studied (Huttner andWeiland, 1997).

Below, the main outcome of the operational experiencesand the research results with regard to nitrogen are pre-sented, as well as more recent results from pilot and dem-onstration plants.

Tab

le3

Was

tew

ater

con

tam

inat

ion

fro

mo

rgan

icw

aste

anae

rob

icd

iges

tio

np

lan

ts(F

rick

eet

al.,

2004

)

Un

itp

aram

eter

Kau

tzan

dN

elle

s(1

994)

Bid

lin

gmai

er(1

995)

Ku

ble

r(1

996)

Lo

ll(1

998)

Bo

nin

get

al.

(199

9)G

raja

and

Wil

der

er(1

999)

;W

idm

ann

(199

9)S

chm

itt

etal

.(2

001)

cG

alle

rtet

al.

(200

2)d

[g/l

][g

/l]

[g/l

][g

/l]

[g/l

]a[g

/l]b

[g/l

][g

/l]

[g/l

]

Fil

tere

dso

lid

mat

ter

4.8–

15.7

No

dat

a9.

6–20

.16

No

dat

aN

od

ata

No

dat

aN

od

ata

No

dat

aN

od

ata

CO

Dto

tal

3.03

–28.

610

.97.

3–28

.32.

28–3

6.2

5.04

10.9

33.

3–23

.8N

od

ata

4.46

–5.8

0C

OD

dis

solv

ed2.

2–2.

5N

od

ata

2.18

–4.9

0N

od

ata

No

dat

aN

od

ata

No

dat

a45

.24.

10–5

.25

BO

D5

0.74

–10.

052.

31.

65–7

.10

0.66

–13.

760.

921.

83.

3–10

.05

No

dat

a1.

88N

tota

lN

od

ata

No

dat

aN

od

ata

No

dat

aN

od

ata

No

dat

a1.

43.

230.

92N

H4–N

0.23

–2.0

0.61

0.51

–2.6

0.57

–1.4

91.

181.

740.

23–0

.98

No

dat

a0.

71

aP

ress

wat

erfr

om

dew

ater

ing

of

am

eso

ph

ilic

anae

rob

icd

iges

tio

np

lan

tfo

rb

io-w

aste

.b

Pre

ssw

ater

fro

md

ewat

erin

go

fa

ther

mo

ph

ilic

anae

rob

icd

iges

tio

np

lan

tfo

rb

io-w

aste

.c

Dew

ater

ing

by

mea

ns

of

asc

rew

pre

ss.

dD

ewat

erin

gb

ym

ean

so

fa

dec

ante

rp

lus

add

itio

no

ffl

occ

ula

nt.


As a consequence of anaerobic digestion, most of thetotal nitrogen in the output of fermentation is present asammonium nitrogen and is organically bound only to asmaller extent. As the nitrogen content limits the recyclabil-ity, as well as the discharge to municipal wastewater treat-ment plants, nitrogen removal will usually be required.Table 5 shows the technical options of reducing the nitro-gen content. Some of the processes have already beenapplied on a large scale for the treatment of industrialand municipal wastewater. The most relevant techniquesand their applicability for the treatment of wastewaterfrom anaerobic digestion are discussed in the followingsections.

Solid matter separation is carried out for the dewateringof solid digestion residues in order to obtain solid materialfor composting or landfilling. Predominantly screw pressesand decanters are used. In addition to that, belt presses canbe applied for further solid matter separation if flocculantsare added to the wastewater. Solid matter separation leadsto a reduction of almost all relevant parameters and is par-ticularly effective for contamination in insoluble, particu-late form. The reduction of nitrogen compounds by solidmatter separation is thus limited, because the most impor-tant nitrogen compounds, such as ammonium, are soluble.Solid matter separation yield differs considerably accordingto the applied aggregate. While screw presses with a screenhave an elimination effect of between 15% and 20%,decanter centrifuges reach a separation effect of between30% and 40% (Huttner and Weiland, 1997; Rexilius,1990).

The membrane technology was originally applied in thebiochemical and pharmaceutical industries for the purifica-tion and recovery of valuable products and raw materialsonly, as the comparatively slow process requires sophisti-cated technology and high investment costs. The use ofthe membrane technology for wastewater treatment is arather recent application.

According to the separated particle size, membranetechnologies are divided into reverse osmosis, nano-filtra-tion, ultra-filtration and micro-filtration. These differ addi-tionally in the required membrane pressure differential(Fig. 4). Initially, mainly membranes on a cellulose basiswere used. As the technology developed, they were substi-tuted step-by-step by natural and synthetic polymers as aresult of their better chemical and mechanical stability.Ceramic and metallic membrane materials, such as glass,play only a subordinate role.

In general, the membrane technology is suitable for theseparation of both organic nitrogen compounds andammonium nitrogen. However, nitrogen compounds areseparated safely only in reverse osmosis plants. The pro-cessing of ‘‘raw’’ wastewater after solid matter separationin a reverse osmosis plant is not possible, as has beenshown by several trial results (trials on wastewater of thebiowaste treatment plants in Ottelfingen und Rumlang,Germany; trials on the treatment of wastewater from anagricultural biogas plant as part of a German federal

Table 4Wastewater and process water contamination from anaerobic digestion stages of mechanical biological waste treatment plants

Process German Minimum requirements for waste water Boning and Doedens (2002) Schulze-Rettmer et al. (1992) Schmitt et al. (2001)

Unit parameter Waste water for mixing(with other effluents),mg/l

Waste water fordischarging in watercourses, mg/l

Mesophilic Thermophilic Un-treatedLiquid digestionresidue, mg/l

Treateda, mg/l 1-stage thermophilic, mg/l

Filtered,mg/l

Centrifuged,mg/l

Filtered,mg/l

Centrifuged,mg/l

Solid matter – – – – – – 2.4 1.6 –CODtotal – 200/400b 3735 12,910 3763 15,534 6830 1023 99,350CODdissolved – – – – – – 2290 780 –BOD5 total – 20 – 1496 – 1978 2447 42 –BOD5 dissolved – – – – – – 427 14 –NH4–N – – – 768 – 1036 1400 21 4153NO3–N – – – – – – – 247 –Ntotal – 70 860 1308 1214 1569 – – –Ptotal – 3 16 60 17 62 – – –Mercury 0.05 – – – – – – – –Lead 0.5 – 0.03 1.2 0.03 14 1.75 0.18 23.5Chrome/Chrome VI 0.5/0.1 – 0.1 0.43 0.13 0.45 0.6 0.1 –Copper 0.5 – 0.11 2.1 0.17 2.1 1.22 0.15 –Cadmium 0.1 – – – – – – – 0.16Nickel 1 – 0.18 0.36 0.22 0.41 0.73 0.33 3.1Zinc 2 – 0.25 9 0.37 9.7 7.45 0.62 57.1Arsenic 0.1 – – – – – – – –Cyanidec 0.2 – – – – – – – –Sulfide 1 – – – – – – – –AOX 0.5 – – 0.59 – 1 – – –Hydrocarbons – 10 – – – – – –

a After nitrification/denitrification and further separation of solid matter.b Limit value for ‘‘indirect discharging’’.c Highly volatile.

K.

Frick

eet

al.

/W

aste

Ma

na

gem

ent

27

(2

00

7)

30

–4

337

Table 5Methods for nitrogen removal from wastewater of biowaste and municipalsolid waste treatment plants

Physical Chemical Biological

Solid matterseparation

Precipitation Nitrification/denitrification

Membranetechnology

Break pointchlorination

AdsorptionDesorption

(stripping)Evaporation


research project on manure treatment and disposal). Thefine dispersed particles still present in the wastewater causerapid membrane fouling, clogging and mechanical damageto the membranes. Therefore, an additional up-streammembrane step has to be added in order to separate finedispersed particles to a large extent.

Results of pilot wastewater treatment in an anaerobicdigestion plant have shown that reverse osmosis alone onlyyields a nitrogen separation effect of about 95%, which isnot sufficient in order to reach the strict German limit valueof 70 mg/l total nitrogen (Table 6). The wastewater treat-

Fig. 4. Classification of membrane and filtration technology, as a fun

Table 6Average content of the permeate in a two-stage reverse osmosis for the treatmand Weiland, 1997)

Parameter Influent [g/kg] Effluent/

Without

COD – 32Ntotal 4.0 190NHx–N 2.9 156NO3–N – Not detePtotal 0.6 <2

ment plant implemented in this case included solid matterseparation by means of a screw press and ultra-filtration.The insufficient reduction of nitrogen can be explained bythe fact that the molecule form of the ammonia is similarto the one of water, which is why retention is insufficient.By adding acids, the ammonia can be transformed intoammonium, which shows a better retention rate. In thiscase, the nitrogen concentration in the discharge could bereduced to less than 10 mg/l. This corresponds to a separa-tion yield of over 99%.

Wastewater treatment by the membrane technologyshows the disadvantage that the produced concentratehas to be subjected either to disposal or to further treat-ment. A further volume reduction of the concentrate canbe reached by evaporation, similar to the process appliedfor leachate treatment. In this case, treatment of theexhaust vapours is indispensable due to the enrichmentwith highly volatile organic compounds.

Sorption is the separation of gaseous components from agas mixture by solvents. Desorption is the inverse processand results in the regeneration of the solvent. The stripping

of ammonia from wastewater is a sorption process widelyapplied in wastewater technology. This method is prefera-bly applied to the purification of industrial wastewaters

ction of pressure (modified according to Baumgarten et al., 1996).

ent of the liquid digestion residue of an agricultural biogas plant (Huttner

permeate of reverse osmosis

addition of acid [mg/l] With addition of acid [mg/l]

<15<10<10

ctable Not detectable<1


contaminated with ammonia, such as from disposal of ani-mal carcasses, or from municipal wastewaters, e.g., fromsewage sludge digestion (Stein et al., 1995; Breitenbucher,1996). The application for the reduction of ammonia con-centrations in agricultural wastewaters has been researchedin pilot plants (Huttner and Weiland, 1997).

The reduction of nitrogen during stripping is limited toammonia, whereas organic nitrogen cannot be stripped.Air or vapour is used as the stripping medium. In the pro-cess flow chart, stripping units are positioned after anaero-bic digestion and solid matter separation, as solid mattercan lead to technical problems in the stripping unit dueto fouling and clogging. Displacement in the dissociationbalance between ammonia/ammonium is achieved by theaddition of alkaline chemicals in the case of air stripping,while during vapour stripping, the temperature is raisedto 100 �C (see Fig. 6).

0

10

20

30

40

50

60

70

80

90

100

7 7.5 8 8.5 9

pH

NH

4-re

du

ctio

n (

%)

NH4-N reduction at 50° C NH4-N re

T

Fig. 5. Ammonium reduction by stripping at different tempe

Fig. 6. Flow scheme for wastewater treatment consisting of an

Vapour or air is supplied in counter flow to the wastewa-ter, where it is charged with ammonia that will then beabsorbed in an acid washing fluid. As a general rule, thestripping air is recycled in order to minimise the odourimpact in the plant surroundings. Sulphuric acid is mostlyused as a washing fluid. In this case, an ammonium sul-phate solution is produced in the stripping unit. In vapourstripping plants with multiplier section, ammonium solu-tions with up to 25% ammonia can be produced. Theammonia in the wet solution can be further processed ina crystallisation stage to solid ammonium hydrogencarbonate.

Due to the requirements on media temperature andpressure, the investment costs for vapour stripping unitsare higher than the costs for air stripping units. As a gen-eral rule, the operational costs of vapour stripping unitsare determined to a great extent by the energy costs for

9.5 10 10.5 11 11.5

Wert

duction at 65° C NH4-N reduction at 35 °C

T = 65°C

= 50°C

T = 35°C

ratures as a function of pH (author’s data, unpublished).

evaporator and an acid scrubber for the exhaust vapours.


vapour production. These costs can be significantly low-ered if reasonable energy recovery concepts are appliedor if free waste heat is available (e.g., from a CHP).Exhaust vapour compression is one of the methods widelyused in practice for energy recovery.

In most cases, the pH value is increased by the additionof lime. The amount of chemicals required to increase thepH is strongly influenced by the composition and the buffercapacity of the wastewater to be treated. Trials with liquidmanure with a high buffer capacity show that it is necessaryto add about 30 kg sodium hydroxide per m3 liquid manurefor increasing the pH value to 11 (Schulze-Rettmer et al.,1990).

The achievable ammonium reduction in the wastewaterdepends on the temperature and the pH value, as presentedin Fig. 5 for ammonia reduction by air stripping in liquiddigestion residue after ultra-filtration. The results havebeen obtained during batch trails on a laboratory scale(batch volume approx. 7 l) at a constant aeration rate of12.9 cm3/min (standard cubic centimetres) and a retentiontime of approx. 3 h.

In summing up it can be said that selection of the strip-ping method, as well as determination of optimum operat-ing parameters, has to take place as a function of:

� the wastewater that has to be treated (its quantity,ammonium concentration, pH value and buffercapacity),� the required reduction effect, or the maximum permissi-

ble ammonium concentration in the output, and� operational aspects, such as space requirements, heat

and chemicals requirements.

The results of trials from air and vapour stripping plantsare compared in Table 7. The concept of the air strippingplant was targeted at the total reduction of the ammonium,while in the vapour stripping plant only partial reduction

Table 7Results of air and vapour stripping trials (Weiland and Harmssen, 1993;Huttner and Weiland, 1997)

Air strippingdesorption column

Vapour stripping

NH4–NInput (mg/l) 3800–4800 2200–3000Through-put

Wastewater (m3/h) 1 3.6Stripping medium(m3 air/h; kg vapour/h)a

3200 max. 450

Temperature (�C) 30–33 100pH (–) 11.5–12.0 7.5–8.0NH4–NOutput (mg/l) 10–30 300–800Reduction yield (%) >99 >80

Absorption columnFluid flow rate (H2SO4) (m3/h)a 5Air flow rate (m3/h) 3200pH (–) 2(NH4)2SO4,Output (%) 33–35

was intended. The reduction yield of the vapour strippingplant can be considerably improved by increasing theamount of the vapour, so that a reduction rate of 99%can be reached, similar to air stripping.

The processes of precipitation and flocculation often takeplace simultaneously, although these are two differentchemical–physical processes. Precipitation means the trans-formation of dissolved, often ionic water compounds intoan undissolved particulate form. Flocculation consists ofthe transformation of fine dispersed insoluble compoundsto larger compounds suitable for mechanical separationby means of flocculation additives.

Ammonium nitrogen can be precipitated as magnesiumammonium phosphate (MAP) according to the followingchemical equation:

NHþ4 þMg2þ þ PO3�4 !MgNH4PO4

For this reaction, the other reactants magnesium and phos-phate must be available in sufficient quantities according tothe respective stoichiometric ratio. By the addition of mag-nesium oxide, this precipitation reaction can be initiated.The optimum pH value for precipitation is between 8.5and 9.5. The purity of the produced MAP depends to agreat extent on the precipitation conditions. Due to the het-erogeneous composition of the wastewaters from anaerobicdigestion, the MAP produced there will usually be contam-inated with other salts.

Up to now, the precipitation of ammonium as MAP hasnot established itself in wastewater treatment. Apart fromthe high technical expenditures, this is mainly due to thehigher costs of the precipitation process in comparison tobiological ammonium reduction, at least if a high removalyield is required, as for example resulting from the Germanlimit value of 30 mg/l. In that case, the revenues from sell-ing the MAP cannot even cover the costs for the usedchemicals (Schulze-Rettmer et al., 1992).

The precipitation of MAP may also occur spontane-ously in the wastewater of anaerobic digestion plants, asgenerally all reactants are available in the wastewater. ThisMAP precipitation can lead to clogging and blocking.Although it is difficult to localise the endangered parts ofthe plant, experience shows that especially areas with highturbulences, such as for example the pipelines, the pumpsor the degasification surfaces, are frequently affected.

In the treatment of wastewater by technical evaporation,the high volatility of the ammonia is used. In technicalevaporation units, the volume of the aqueous solution isreduced by evaporation; the contaminants remain in theconcentrate. As a general rule, the distillate shows lowamounts of dissolved matter and low COD (chemical oxy-gen demand) values, the COD being mainly caused by highvolatile organic components. Evaporation units are usuallycharacterised by a high thermal energy demand. Further-more, technical problems in the operation of evaporationunits may occur due to the formation of films and foulingof the heat transfer units. These films are caused by cakingof organic compounds, which are sensitive to temperature,

Table 9Results from biological treatment of the wastewater of an organic wastetreatment plant (Kubler, 1996)

Unit parameter Untreated [mg/l] After flocculationand sedimentation[mg/l]

After biologicaltreatment[mg/l]

Filteredsolid matter

5270–20,610 1390–2010 50–100

CODtotal 10,000–28,300 3000–5520 900–1380CODsoluble 2840–3830 1850–2610 820–1190BOD5 4290 1450 80NHx–N – 437–1142 1–35


or by the crystallisation of dissolved matter on the walls ofthe evaporation unit. As a consequence of fouling, theevaporation efficiency is decreased.

Depending on the configuration of the evaporation unit,the ammonium can be left in the concentrate or can bestripped with the vapour. The latter happens with molecu-lar ammonia, which is then, after stripping, left in the con-densate. Afterwards, the ammonia can also be strippedfrom the vapour as ammonium sulphate or ammoniumnitrate with the aid of sulphuric or nitric acid in an addi-tional acid scrubber. If acid is added to the wastewaterinput into the evaporation unit, ammonia will be trans-formed to ammonium, which is then – during evaporation– left in the concentrate (Huttner et al., 1996).

The evaporation of wastewater from agricultural biogasplants has been analysed in connection with various demon-stration-scale research projects. Vacuum falling film evapo-rators and vacuum horizontal spraying film evaporatorswere used, as these are highly suitable for the use of excessthermal energy that frequently is available at biogas plants.In the case studied here, excess heat with a temperature ofabout 70 �C was used. In one plant, ammonium retentionwas achieved by lowering the pH in the wastewater input;during evaporation, the ammonium was thus left in the con-centrate. In the other plant, molecular ammonia wasstripped with the vapour and then eliminated in an acidscrubber. With both plant configurations, the achievedammonium retention effects were about 90%. Furtherincreases in the retention yield seem to be possible, but werenot required in the studied case. Furthermore, these trialsshow that with both plant configurations organic nitrogencompounds remain completely in the concentrate (Table 8).

The biological elimination of ammonium takes places bynitrification/denitrification of the nitrogen. The degradationof ammonium nitrogen to elemental nitrogen occurs by theintermediary step of nitrification according to the followingchemical equation:

NHþ4 þ 2O2 ! NO�3 þ 2H2Oþ 2Hþ

with the aid of Nitrosomonas and Nitrobacter, which areammonium oxidising micro-organisms. Here, nitrate isformed via nitrite formation. The latter, however, takesplace so rapidly that usually only low residual concentra-tions of nitrite are measured. In order to prevent the pro-cess from being inhibited due to oxygen limitation, anoperational oxygen content of more than 2 mg O2/l is re-

Table 8Material characteristics of the input and the exhaust vapour condensate ofa plant for evaporation of the wastewater from an agricultural biogasplant (KTBL, 1999)

Parameter Input evaporator Exhaust vapourcondensate

COD (mg/l) No data 3100Ntotal (mg/l) 5700 40NHx–N (mg/l) 3800 40pH-value 6.8 3.3

quired. Furthermore, the produced hydrogen ions mustbe buffered.

The degradation of nitrate to elemental nitrogen takesplace during denitrification under anoxic conditions,according to the chemical equation:

NO�3 þ 1=2H2O! 1=2N2 þ 5=2OþOH�

In order to achieve a high elimination effect, the environ-mental conditions have to be adjusted according to theneeds of the involved microorganisms. In addition to anadequate oxygen content for the nitrification step, thiscomprises a pH value in between 7 and 8, as well as theavailability of an easily accessible carbon source for thedenitrification step. Furthermore, a stable biocenosis hasto be formed, which is adapted to the specific nitrogenconcentration.

Up to now, biological nitrogen removal from wastewa-ter of solid waste treatment plants has mainly been studiedin experimental and pilot plants (Table 9). In that context,the ammonium reduction yield is limited, as easily degrad-able carbon sources are normally lacking in the wastewaterafter anaerobic digestion. Thus, denitrification is ofteninhibited as long as no additional carbon source is added(Kubler, 1996).

This statement is also confirmed by more recent researchresults on nitrification/denitrification for treatment of theexcess water resulting from anaerobic digestion of organi-cally contaminated process water (Table 4) (Santen et al.,2003). The results show that the ammonium concentrationin the process water can be reduced to up to 99% throughbiological treatment. However, in the treated wastewater,residual nitrate concentrations of 250 mg/l were detected,indicating insufficient denitrification. This inhibition ofdenitrification may be caused by inadequate process condi-tions as, for instance, aerobic conditions in the anoxiczones for denitrification. Nevertheless, this is more proba-bly a consequence of the lacking carbon source in thewastewater, which has already undergone anaerobicdigestion.

5. Summary

In the present paper, the operational problems causedby nitrogen compounds in anaerobic digestion and in the


peripheral technology are explained. Different nitrogencompounds, such as ammonium, dinitrous oxide as wellas nitrite and nitrate, can inhibit the biological process ormake it difficult to comply with the corresponding emissionlimit values.

In the exhaust air, in particular ammonia, dinitrousoxide and odour play an important role. While ammoniacontributes to the odour emissions and has adverse effectson humans, dinitrous oxide plays an important role inthe anthropogenic greenhouse effect. Thus, these compo-nents should be reduced in the exhaust air. Experience withthe treatment of exhaust air from aerobic post-treatment ofsolid anaerobic digestion residues shows that, with an acidscrubber and subsequent regenerative thermal oxidation(RTO), even the strict German limit values on exhaustair contamination can be met. In the case of biowaste treat-ment, fairly simple exhaust air treatment comprising anacid scrubber for ammonia elimination and a subsequentbio-filter for the reduction of odour, can be regarded tobe sufficient.

Unlike ‘‘classical’’ aerobic biological waste treatment,relevant amounts of process and wastewater are generatedduring anaerobic digestion. As internal recycling of theprocess water is limited due to the contamination with var-ious organic and inorganic compounds, and as there willalways be some excess water, all anaerobic processes haveto integrate some solution for the treatment or disposalof wastewater. In particular, nitrogen compounds limitthe options for the recycling or discharge of the processwater, so that as a general rule nitrogen elimination fromthe process water is required.

However, up to now the methods for nitrogen removalknown from industrial and communal wastewater treat-ment are rarely applied in anaerobic digestion plants. Untilnow, the excess water from organic waste treatment plantshas been used predominantly in agriculture or has been dis-charged to communal wastewater treatment plants. Neitheris there any experience available with the treatment of pro-cess water from municipal solid waste treatment plants.Nevertheless, results from different research work and pilotscale applications show that established treatment technol-ogies, such as reverse osmosis, evaporation and ammoniastripping, can also be applied for the treatment of processand excess waters from anaerobic digestion. In comparisonto that, biological nitrogen elimination by nitrification/denitrification is in this case not that suitable, as denitrifi-cation is often inhibited by the lack of an easily availablecarbon source.

All in all, the material characteristics of the processand excess waters from anaerobic digestion differ consid-erably from the ones of communal and industrial waste-waters. Therefore, a simple one-to-one transfer oftechnologies, design and operation knowledge from com-munal and industrial wastewater treatment is as a rulenot enough. In fact, the results and the efficiency rateof the selected technology must be tested in individualcases.

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